To meet growing demands in recent years for broadband Internet-connected devices, there have been provided various cable modems that can be connected to the Internet via CATV. It has been desirable to reduce the size of cable modem devices as with other electronic devices. For reducing the size of cable modem devices, tuners for use in the cable modem devices need to be reduced in size of necessity. Though there were limitations on efforts to reduce the size of such tuners because they were made up of discrete parts, it has been made possible in recent years to greatly reduce the size of tuners because of the advanced circuit integration technology.
Most tuner ICs for use in the cable modem applications employ a double superheterodyne reception configuration. FIG. 8 of the accompanying drawings is a block diagram of a tuner of double superheterodyne reception. In FIG. 8, the tuner has an antenna 101, a variable-gain amplifier 102 for amplifying a CATV signal, a first voltage-controlled oscillator 103 for locally oscillating at a frequency depending on a control voltage applied thereto, a first frequency mixer 104 for multiplying the CATV signal output from the variable-gain amplifier 102 and the output signal from first voltage-controlled oscillator 103 for frequency conversion to output a first intermediate-frequency signal having a frequency corresponding to the difference between the multiplied signals, a bandpass filter 105 for passing a signal in a given frequency band only to extract an intermediate-frequency signal in a desired channel thereby to select a station, a second voltage-controlled oscillator 106 for locally oscillating at a frequency depending on a control voltage applied thereto, a second frequency mixer 107 for multiplying the first intermediate-frequency signal output from the bandpass filter 105 and the output signal from the second voltage-controlled oscillator 106 for frequency conversion to output a second intermediate-frequency signal having a frequency corresponding to the difference between the multiplied signals, and an intermediate-frequency signal amplifier 108 for amplifying the second intermediate-frequency signal output from the second frequency mixer 107 and outputting the amplified second intermediate-frequency signal as a tuner output signal.
Operation and characteristics of the tuner will briefly be described below.
A CATV signal having a frequency of 100 MHz, for example, which is input from the antenna 101, is amplified by the variable-gain amplifier 102, and sent to the first frequency mixer 104. The first frequency mixer 104 is supplied with the CATV signal and a local oscillation signal having a frequency of 1300 MHz, for example, output from the first voltage-controlled oscillator 103, and performs frequency conversion to output a first intermediate-frequency signal having a frequency of 1200 MHz. The bandpass filter 105 passes a signal having a frequency near 1200 MHz to extract a first intermediate-frequency signal in a desired channel. The second frequency mixer 107 is supplied with the first intermediate-frequency signal having the frequency of 1200 MHz and a local oscillation signal having a frequency of 1156 MHz, for example, output from the second voltage-controlled oscillator 106, and performs frequency conversion to output a second intermediate-frequency signal having a frequency of 44 MHz. The intermediate-frequency signal amplifier 108 amplifies the second intermediate-frequency signal supplied thereto and outputs the amplified second intermediate-frequency signal as a tuner output signal. The above frequencies of the CATV signal, the first intermediate-frequency signal, the second intermediate-frequency signal, etc. are given as an example of frequencies that are actually employed in a CATV system tuner.
The tuner that performs the above frequency conversion has an input signal level ranging from −70 to +30 dBm. Since signals of a maximum of 130 waves are input to the tuner, the variable-gain amplifier 102 at the first stage is required to have a gain of 10 dB, a noise figure of 6 dB, a maximum attenuation level of 40 dB, a third-order Input Intercept Point (hereinafter referred to as “IIP3”) of +15 dBm at a maximum gain, and an IIP3of +30 dBm at an attenuated gain (−15 dB). Therefore, as the gain is lower, stricter distortion requirements are imposed on the variable-gain amplifier 102.
The variable-gain amplifier with such distortion requirements placed thereon includes bipolar transistors and dual-gate field-effect transistors (hereinafter referred to as “dual-gate FETs”). Operation of a dual-gate FET will be described below. FIG. 9 is a circuit diagram of a variable-gain amplifier including a dual-gate FET. In FIG. 9, the variable-gain amplifier has a first FET 111, a second FET 112 connected in cascade to the first FET 111, a voltage source 113, a ground level 114, a signal input terminal 115, a resistor 116 connected to the gate G1 of the first FET 111 for applying a suitable bias voltage to an input signal, a voltage source 117 for applying a suitable bias voltage, a variable voltage source 118 connected to the gate G2 of the second FET 112, a load resistor 119 for extracting an output signal, and a signal output terminal 120. The voltage source 117 may be provided by dividing the voltage from the voltage source 113 with resistors.
In the dual-gate FET of the above construction, an input signal is applied to the gate G1 of the first FET 111, and an output signal is produced from the drain of the second FET 112. The gain of the variable-gain amplifier is controlled by changing the voltage applied to the gate T2 of the second FET 112. For example, when the voltage applied to the gate G2 of the second FET 112 is lowered, the source voltage of the second FET 112 is lowered, reducing a drain-to-source voltage Vds of the first FET 111. As the drain-to-source voltage Vds becomes lower, the mutual conductance gm of the first FET 111 becomes smaller, resulting in a reduction in the gain. Conversely, when the voltage applied to the gate G2 of the second FET 112 is increased, the gain is increased. The variable-gain amplifier operates in the same manner as described above even if the gain-control FET 112 is replaced with an NPN transistor. A variable-gain amplifier including a dual-gate FET is disclosed in Japanese Patent Laid-open No. 2002-176371 (Paragraph 0010, FIG. 1).
Characteristics about gain, noise, etc. of the variable-gain amplifier including the dual-gate FET shown in FIG. 9 will be described below. FIG. 10 of the accompanying drawings is a plan view showing a general arrangement of an FET formed on an integrated circuit (IC). In FIG. 10, Lg represents a gate length, and Wg a gate width. FIG. 11 of the accompanying drawings is a graph showing the relationship between the gate width, the gain, and the noise figure. In FIG. 11, PG represents the gain, and NF the noise figure. FIG. 12 of the accompanying drawings is a graph showing the relationship between the gain attenuation and the IIP3. In FIG. 12, the gain attenuation and the IIP3are related in different patterns depending on the gate width used as a parameter. As can be seen from FIGS. 11 and 12, as the gate width Wg increases, the gain PG increases and the noise figure NF decreases, increasing noise characteristics. However, distortion characteristics given as the IIP3are lowered. In order to fulfill the requirements for the PG and NF characteristics when the gain is maximum, it is necessary to use FETs with Wg=20 (μm), which however fail to fulfill the requirements for the characteristics of the IIP3when the gain is attenuated. In order to fulfill the requirements for the characteristics of the IIP3when the gain is attenuated, it is necessary to use FETs with Wg=5 (μm), which lower the PG and NF characteristics. This means that there is a trade-off between efforts to increase the PG and NF characteristics and efforts to increase the IIP3characteristics, and it is difficult to obtain a variable-gain amplifier satisfying both the requirements for the PG and NF characteristics and the requirements for the IIP3characteristics.
The present invention has been made in order to solve the above problems. It is an object of the present invention to provide a variable-gain amplifier capable of improving the distortion characteristics IIP3when the gain is attenuated, without lowering the characteristics with respect to the gain PG and the noise figure NF when the gain is maximum.